Lenses used in digital cameras typically include glass lenses and plastic lenses. Plastic lenses are generally produced by an injection mold. In general, an injection mold used for an injection molding machine includes a mold body including a first template and a second template joined to each other in such a manner as to allow their separation. The mold body defines a cavity (a space which can be filled with molten plastic) of a configuration corresponding to that of an article to be molded, a flow passage through which molten plastic can flow through toward the cavity, and a gate through which the molten plastic flowing through the plastic flow passage can be injected into the cavity. At least one of the first and second templates is supported in such a manner as to be displaceable in a direction perpendicular to a parting face or line therebetween, so that relative displacement between the first template and the second template permits the mold body to be opened.
During the molding process, the first and second templates are pressed against each other and then clamped together using any suitable clamping mechanism to close the mold body. Then, molten plastic is injected through the gate into the cavity, resulting in it being molded into a molded article. After the molding is finished, the first and second templates are separated from each other, so that the molded article may be removed from the mold body.
Referring to FIG. 1, a typical injection mold 10 for molding plastic lenses includes an upper molding body 11 and a lower molding body 13. The upper molding body 11 defines a main runner 111 in a middle portion thereof. The injection mold defines two sub-runners 131 and two molding cavities 133 between the upper molding body 11 and the lower molding body 13.
In use, molten plastic is injected into the two molding cavities 133 through the main runner 111 and the two sub-runners 131. Because the main runner 111 is defined in the upper molding body 11, the molten plastic should be injected into the two molding cavities 133 from one side thereof. As a result, the molten plastic in the molding cavities 133 has an asymmetrical rate of cooling, and plastic lenses formed in the molding cavities 133 are asymmetrical as a result. In addition, gate (i.e. extra matter) formed in the sub-runners 131 connects the plastic lenses, and this gate must be cut off with a cutting machine. Cutting off the gate from the plastic lenses is tends to adversely impact the precision of the plastic lenses. Plastic lenses require precision molding, therefore, in order to mold each lens to a high degree of precision, it is important that the molten plastic is evenly and reliably fed into every corer of the cavity without leaving gate in the sub-runners 131.
Accordingly, what is needed is a new injection molding device, which is able to mold an accurate lens without influencing the lens configuration.